gh-138361: Add N format unit for non-negative Py_ssize_t

Doc/c-api/arg.rst

@@ -285,6 +285,9 @@ the minimal value for the corresponding signed integer type of the same size.
 ``n`` (:class:`int`) [:c:type:`Py_ssize_t`]
    Convert a Python integer to a C :c:type:`Py_ssize_t`.
 
+``N`` (:class:`int`) [:c:type:`Py_ssize_t`]
+   Convert a non-negative Python integer to a C :c:type:`Py_ssize_t`.
+
 ``c`` (:class:`bytes` or :class:`bytearray` of length 1) [char]
    Convert a Python byte, represented as a :class:`bytes` or
    :class:`bytearray` object of length 1, to a C :c:expr:`char`.

Lib/test/test_capi/test_getargs.py

@@ -290,6 +290,32 @@ def test_I(self):
         with self.assertWarns(DeprecationWarning):
             self.assertEqual(UINT_MAX & -VERY_LARGE, getargs_I(-VERY_LARGE))
 
+    def test_N(self):
+        from _testcapi import getargs_N
+        # n returns 'Py_ssize_t', and does range checking
+        # (0 ... PY_SSIZE_T_MAX)
+        self.assertRaises(TypeError, getargs_N, 3.14)
+        self.assertEqual(99, getargs_N(Index()))
+        self.assertEqual(0, getargs_N(IndexIntSubclass()))
+        self.assertRaises(TypeError, getargs_N, BadIndex())
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(1, getargs_N(BadIndex2()))
+        self.assertEqual(0, getargs_N(BadIndex3()))
+        self.assertRaises(TypeError, getargs_N, Int())
+        self.assertEqual(0, getargs_N(IntSubclass()))
+        self.assertRaises(TypeError, getargs_N, BadInt())
+        self.assertRaises(TypeError, getargs_N, BadInt2())
+        self.assertEqual(0, getargs_N(BadInt3()))
+
+        self.assertRaises(OverflowError, getargs_N, PY_SSIZE_T_MIN-1)
+        self.assertRaises(OverflowError, getargs_N, PY_SSIZE_T_MIN)
+        self.assertRaises(OverflowError, getargs_N, -1)
+        self.assertEqual(PY_SSIZE_T_MAX, getargs_N(PY_SSIZE_T_MAX))
+        self.assertRaises(OverflowError, getargs_N, PY_SSIZE_T_MAX+1)
+
+        self.assertEqual(42, getargs_N(42))
+        self.assertRaises(OverflowError, getargs_N, VERY_LARGE)
+
     def test_k(self):
         from _testcapi import getargs_k
         # k returns 'unsigned long', no range checking

Misc/NEWS.d/next/C_API/2025-09-01-22-45-45.gh-issue-138361.okpmAJ.rst

@@ -0,0 +1,2 @@
+Added ``N`` as new ``PyArg_Parse`` format unit for non-negative ``Py_ssize_t``
+values.

Modules/_testcapi/getargs.c

@@ -437,6 +437,16 @@ getargs_n(PyObject *self, PyObject *args)
     return PyLong_FromSsize_t(value);
 }
 
+static PyObject *
+getargs_N(PyObject *self, PyObject *args)
+{
+    Py_ssize_t value;
+    if (!PyArg_ParseTuple(args, "N", &value)) {
+        return NULL;
+    }
+    return PyLong_FromSsize_t(value);
+}
+
 static PyObject *
 getargs_p(PyObject *self, PyObject *args)
 {
@@ -793,6 +803,7 @@ static PyMethodDef test_methods[] = {
     {"getargs_keywords", _PyCFunction_CAST(getargs_keywords), METH_VARARGS|METH_KEYWORDS},
     {"getargs_l",               getargs_l,                       METH_VARARGS},
     {"getargs_n",               getargs_n,                       METH_VARARGS},
+    {"getargs_N",               getargs_N,                       METH_VARARGS},
     {"getargs_p",               getargs_p,                       METH_VARARGS},
     {"getargs_positional_only_and_keywords", _PyCFunction_CAST(getargs_positional_only_and_keywords), METH_VARARGS|METH_KEYWORDS},
     {"getargs_s",               getargs_s,                       METH_VARARGS},

Python/getargs.c

@@ -814,6 +814,27 @@ convertsimple(PyObject *arg, const char **p_format, va_list *p_va, int flags,
         *p = ival;
         break;
     }
+
+    case 'N': { /* Py_ssize_t - non negative */
+        PyObject *iobj;
+        Py_ssize_t *p = va_arg(*p_va, Py_ssize_t *);
+        Py_ssize_t ival = -1;
+        iobj = _PyNumber_Index(arg);
+        if (iobj != NULL) {
+            ival = PyLong_AsSsize_t(iobj);
+            Py_DECREF(iobj);
+        }
+        if (ival == -1 && PyErr_Occurred())
+            RETURN_ERR_OCCURRED;
+        else if (ival < 0) {
+            PyErr_SetString(PyExc_OverflowError,
+                            "integer is less than minimum");
+            RETURN_ERR_OCCURRED;
+        }
+        *p = ival;
+        break;
+    }
+
     case 'l': {/* long int */
         long *p = va_arg(*p_va, long *);
         long ival = PyLong_AsLong(arg);
@@ -2618,6 +2639,7 @@ skipitem(const char **p_format, va_list *p_va, int flags)
     case 'L': /* long long */
     case 'K': /* long long sized bitfield */
     case 'n': /* Py_ssize_t */
+    case 'N': /* Py_ssize_t - non negative */
     case 'f': /* float */
     case 'd': /* double */
     case 'D': /* complex double */